Fix missing code example in changelog for 8.4.1

(cherry picked from commit 3ae408cf93d10b9a2e5acb4186dc8e0c62e0afb6)

docs/users_guide/8.4.1-notes.rst

@@ -186,7 +186,7 @@ Compiler
     ``error``: ::
 
       instance Show (Empty a) where
-        showsPrec = "Void showsPrec"
+        showsPrec = error "Void showsPrec"
 
     Now, they emit code that inspects the argument. That is, if the argument
     diverges, then showing it will also diverge: ::